CVE-2023-6449: Contact Form 7 Auth Bypass Vulnerability

CVE-2023-6449 Overview

The Contact Form 7 plugin for WordPress contains an arbitrary file upload vulnerability due to insufficient file type validation in the validate function and inadequate blocklisting in the wpcf7_antiscript_file_name function. This vulnerability affects versions up to and including 5.8.3, allowing authenticated attackers with editor-level capabilities or higher to upload arbitrary files to the affected site's server.

While the default .htaccess configuration typically prevents direct remote code execution, the vulnerability remains significant because uploaded files may persist on the server when combined with other plugins. This persistence, combined with another vulnerability such as local file inclusion (LFI), could lead to remote code execution.

Critical Impact

Authenticated attackers with editor privileges can upload arbitrary files, potentially achieving remote code execution when chained with additional vulnerabilities like local file inclusion.

Affected Products

• Contact Form 7 versions up to and including 5.8.3
• WordPress installations using vulnerable Contact Form 7 plugin
• rocklobster contact_form_7

Discovery Timeline

• December 1, 2023 - CVE-2023-6449 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-6449

Vulnerability Analysis

This arbitrary file upload vulnerability (CWE-434: Unrestricted Upload of File with Dangerous Type) exists within the Contact Form 7 plugin's file handling mechanisms. The vulnerability stems from two separate weaknesses: the validate function performs insufficient file type validation, and the wpcf7_antiscript_file_name function implements an inadequate blocklist approach that can be bypassed.

When exploited, authenticated users with editor-level access or above can upload files that would normally be restricted. The WordPress .htaccess configuration provides a defense-in-depth measure that prevents direct execution of uploaded files in most standard configurations. However, by default, uploaded files are immediately deleted from the server after processing, which limits the exploitation window.

The real danger emerges when other plugins on the same WordPress installation alter this default behavior, allowing uploaded files to persist on the server. In these scenarios, attackers can chain this file upload vulnerability with local file inclusion or similar vulnerabilities to achieve remote code execution.

Root Cause

The root cause lies in the flawed file validation logic within Contact Form 7's file upload handling. The validate function does not adequately verify file types before processing, and the wpcf7_antiscript_file_name function relies on blocklisting rather than allowlisting to prevent malicious file uploads. This approach is inherently vulnerable because it attempts to enumerate all potentially dangerous file extensions rather than explicitly permitting only safe ones.

The vulnerable code is located in the includes/formatting.php file at line 275 in version 5.8.3, where the antiscript filename validation fails to account for all potentially dangerous file extensions that could be abused by attackers.

Attack Vector

The attack vector requires network access with authenticated credentials at the editor level or higher. An attacker must:

1. Authenticate to the WordPress site with at least editor privileges
2. Submit a crafted file through a Contact Form 7 form that bypasses the insufficient blocklist validation
3. Ensure the file persists on the server (either through another plugin's behavior or timing exploitation)
4. Chain with another vulnerability such as local file inclusion to execute the uploaded malicious file

The technical details of the vulnerability and the fix can be found in the WordPress Changeset for Plugin and the GitHub Version Comparison between versions 5.8.3 and 5.8.4.

Detection Methods for CVE-2023-6449

Indicators of Compromise

• Unexpected files appearing in WordPress upload directories, particularly with unusual or double extensions
• Web server logs showing file upload attempts to Contact Form 7 endpoints from editor-level accounts
• Presence of executable file types (.php, .phtml, .php5, etc.) in form upload directories
• Unusual form submission patterns from privileged user accounts

Detection Strategies

• Monitor file system events for creation of executable files in WordPress upload directories
• Implement web application firewall (WAF) rules to detect file upload attempts with suspicious extensions
• Audit WordPress user activity logs for editor-level accounts performing unusual file submissions
• Deploy SentinelOne Singularity Platform to detect and block malicious file creation and execution attempts on web servers

Monitoring Recommendations

• Enable detailed logging for Contact Form 7 plugin activities
• Configure file integrity monitoring on WordPress installations to detect unauthorized file additions
• Implement real-time alerting for new file creations in upload directories with executable extensions
• Regularly audit installed WordPress plugins for known vulnerabilities using automated scanning tools

How to Mitigate CVE-2023-6449

Immediate Actions Required

• Update Contact Form 7 to version 5.8.4 or later immediately
• Audit all WordPress user accounts with editor-level access or higher for unauthorized activity
• Review uploaded files in WordPress directories for any suspicious or unexpected content
• Consider implementing additional file upload restrictions at the web server level

Patch Information

Rocklobster has released Contact Form 7 version 5.8.4 which addresses this vulnerability. The patch improves file type validation in the validate function and strengthens the blocklisting mechanism in wpcf7_antiscript_file_name. Details of the security fix are available in the Contact Form 7 Release Note and the Wordfence Vulnerability Report.

Workarounds

• Temporarily disable file upload fields in Contact Form 7 forms until the patch can be applied
• Restrict editor-level access to trusted users only and audit existing editor accounts
• Implement server-side rules to block upload of executable file types regardless of plugin validation
• Ensure .htaccess rules are properly configured to prevent script execution in upload directories

# Example .htaccess configuration to prevent PHP execution in uploads directory
# Place in wp-content/uploads/.htaccess
<FilesMatch "\.(?i:php|php3|php4|php5|phtml|pl|py|cgi)$">
    Deny from all
</FilesMatch>

# Alternative for Apache 2.4+
<FilesMatch "\.(?i:php|php3|php4|php5|phtml|pl|py|cgi)$">
    Require all denied
</FilesMatch>